a. Field of the Invention
The invention relates to electrolytes for batteries. In particular, the invention relates to lithium-ion electrolytes for lithium-ion batteries.
b. Background Art
Lithium-ion (“Li-ion”) cells typically include a carbon (e.g., coke or graphite) anode intercalated with lithium ions to form LixC; an electrolyte consisting of a lithium salt dissolved in one or more organic solvents; and a cathode made of an electrochemically active material, typically an insertion compound, such as LiCoO2. During cell discharge, lithium ions pass from the carbon anode, through the electrolyte to the cathode, where the ions are taken up with the simultaneous release of electrical energy. During cell recharge, lithium ions are transferred back to the anode, where they reintercalate into the carbon matrix.
Future NASA missions aimed at exploring Mars, the Moon, and the outer planets require rechargeable batteries that can operate effectively over a wide temperature range (−60° C. (Celsius) to +60° C. (Celsius)) to satisfy the requirements of various applications, including: Landers (lander spacecraft), Rovers (surface rover spacecraft), and Penetraters (surface penetrator spacecraft). Some future applications typically will require high specific energy batteries that can operate at very low temperatures, while still providing adequate performance and stability at higher temperatures. In addition, many of these applications envisioned by the ESRT (Exploration Systems Research and Technology) program will require improved safety, due to their use by humans. Lithium-ion rechargeable batteries have the demonstrated characteristics of high energy density, high voltage, and excellent cycle life. Currently, the state-of-the-art lithium-ion system has been demonstrated to operate over a wide range of temperatures (−40° C. to +40° C.), however, abuse conditions such as being exposed to high temperature, overcharge, and external shorting, can often lead to cell rupture and fire. The nature of the electrolyte can greatly affect the propensity of the cell/battery to catch fire, given the flammability of the organic solvents used within. Therefore, extensive effort has been devoted recently to developing non-flammable electrolytes to reduce the flammability of the cell/battery.
Desired properties for Li-ion electrolytes can include high conductivity over a wide temperature range (e.g., 1 mS (milli-Siemens) cm−1 from −60° C. to +60° C.); good electrochemical stability over a wide voltage range (e.g., 0 to 4.5V (volts)) with minimal oxidative degradation of solvents/salts; good chemical stability; good compatibility with a chosen electrode couple, including good SEI (solid electrolyte interface) characteristics on the electrode and facile lithium intercalation/de-intercalation kinetics; good thermal stability; good low temperature performance throughout the life of the cell, including good resilience to high temperature exposure and minimal impedance build-up with cycling and/or storage; and low toxicity. Since the flammability of the electrolyte solution in Li-ion batteries is a major concern, significant research has been devoted to developing electrolyte formulations with increased safety. Known electrolytes used in state-of-the-art Li-ion cells have typically comprised binary mixtures of organic solvents, for example, high proportions of ethylene carbonate, propylene carbonate or dimethyl carbonate, within which is dispersed a lithium salt, such as lithium hexafluorophosphate (LiPF6). Examples may include 1.0 M (molar) LiPF6 in a 50:50 mixture of ethylene carbonate/dimethyl carbonate, or ethylene carbonate/diethyl carbonate. More recently, electrolytes have also been developed which combine more than two solvents and/or have incorporated the use of electrolyte additives to address specific performance goals.
Fluorinated esters have been incorporated into multi-component electrolyte formulations and their performance was demonstrated over a wide temperature range (−60° C. to +60° C.) (see U.S. application Ser. No. 12/419,473 filed Apr. 7, 2009, for “Lithium Ion Electrolytes and Lithium Ion Cells with Good Low Temperature Performance”, Smart et al.). The fluorinated ester co-solvents were employed due to their favorable properties and improved safety characteristics, mainly associated with their low flammability associated with their halogenated nature. Since the flammability of the electrolyte solution in Li-ion batteries is a major concern, significant research has been devoted to developing electrolyte formulations with increased safety. To achieve this, a number of approaches have been adopted, including the use of low-flammability solvents and the use of electrolyte additives. Regarding the first approach, the use of halogenated solvents (Smart, et al., “Improved Performance of Lithium-Ion Cells with the use of Fluorinated Carbonate-Based Electrolytes”, Journal of Power Sources, 119-121, 359-367 (2003)) and ionic liquids (Xu, Kang, “Nonaqueous Liquid Electrolytes for Lithium-Based Rechargeable Batteries”, Chemical Reviews, 104, 4303-4417 (2004)) have been pursued. With respect to the use of electrolyte additives, the main focus has been upon the use of phosphorus containing additives, including trimethyl phosphate (Wang, et al., “Nonflammable Trimethyl Phosphate Solvent-Containing Electrolytes for Lithium-Ion Batteries, I. Fundamental Properties”, Journal of the Electrochemical Society, 148(10), A1058-A1065 (2001); Wang, et al., “Nonflammable Trimethyl Phosphate Solvent-Containing Electrolytes for Lithium-Ion Batteries, II. The Use of An Amorphous Carbon Anode”, Journal of the Electrochemical Society, 148(10), A1066-A1071 (2001)), triethyl phosphate (Xu, et al., “An Attempt to Formulate Nonflammable Lithium Ion Electrolytes with Alkyl Phosphates and Phosphazenes”, Journal of the Electrochemical Society, 149(5), A622-A626 (2002)), triphenyl phosphate (Doughty, et al., “Effects of Additives on Thermal Stability of Li ion Cells”, Journal of Power Sources, 146(1-2), 116-120 (2005)), tris(2,2,2-trifluoroethyl)phosphate (Xu, et al., “Nonflammable Electrolytes for Li-Ion Batteries Based on a Fluorinated Phosphate”, Journal of the Electrochemical Society, 149(8), A1079-A1082 (2002); Xu, et al., “Evaluation of Fluorinated Alkyl Phosphates as Flame Retardants in Electrolytes for Li-Ion Batteries, I. Physical and Electrochemical Properties”, Journal of the Electrochemical Society, 150(2), A161-A169 (2003)), and bis(2,2,2-trifluoroethyl)methyl phosphonate (TFMPo) (Xu, et al., “Nonflammable Electrolytes for Li-Ion Batteries Based on a Fluorinated Phosphate”, Journal of the Electrochemical Society, 149(8), A1079-A1082 (2002); Xu, et al., “Evaluation of Fluorinated Alkyl Phosphates as Flame Retardants in Electrolytes for Li-Ion Batteries, I. Physical and Electrochemical Properties”, Journal of the Electrochemical Society, 150(2), A161-A169 (2003)).
In addition, known improvements have been made to the safety characteristics of Li-ion electrolytes by the addition of flame retardant additives, such as triphenyl phosphate (referred to as TPhPh or TPP or TPPa), tributyl phosphate (referred to as TBP or TBuPh), triethyl phosphate (referred to as TEP or TEtPh), and bis(2,2,2-trifluoroethyl)methyl phosphonate (referred to as BTFEMP or TFMPo) (see NPO-46262, May 8, 2008). A number of electrolytes based upon these approaches have delivered good performance over a wide temperature range, good cycle life characteristics, and improved safety characteristics, namely reduced flammability. Since the flammability of the electrolyte solution in Li-ion batteries is a major concern, significant research has been devoted to developing electrolyte formulations with increased safety. To achieve this, a number of approaches have been adopted, including the use of low-flammability solvents and the use of electrolyte additives. As discussed above, regarding the first approach, the use of halogenated solvents (Smart, et al., “Improved Performance and Safety of Lithium Ion Cells with the Use of Fluorinated Carbonate-Based Electrolytes”, Journal of Power Sources, 119-12, 359-367 (2003)) and ionic liquids (Xu, Kang, “Nonaqueous Liquid Electrolytes for Lithium-Based Rechargeable Batteries”, Chemical Review, 104(10), 4303-4417 (2004)) have been pursued. With respect to the use of electrolyte additives, the main focus has been upon the use of phosphorus containing additives, including trimethyl phosphate, triethyl phosphate, triphenyl phosphate, tris(2,2,2-trifluoroethyl)phosphate, and bis(2,2,2-trifluoroethyl)methyl phosphonate (TFMPo) (see all above). In addition, known phosphorus based flame retardant additives have been investigated. Some of these electrolyte additives have been demonstrated to perform well in multi-component Li-ion battery electrolytes. For example, tris(2,2,2-trifluoroethyl)phosphate (see FIG. 5e) has been used in electrolyte solutions consisting of 1.0M LiPF6 in EC+EMC (1:1 wt %) in varying concentrations (Xu, et al., “Nonflammable Electrolytes for Li-Ion Batteries Based on a Fluorinated Phosphate”, Journal of the Electrochemical Society, 149(8), A1079-A1082 (2002); Xu, et al., “Evaluation of Fluorinated Alkyl Phosphates as Flame Retardants in Electrolytes for Li-Ion Batteries”, Journal of the Electrochemical Society, 150(2), A161-A169 (2003)). Tris(2,2,2-trifluoroethyl)phosphite (see FIG. 5f) has been used in concentrations of up to 15% in solutions of 1.0M LiPF6 PC+EC+EMC (3:3:4 wt %) (Zhang, et al., “Tris(2,2,2-trifluoroethyl)Phosphite as a Co-Solvent for Nonflammable Electrolytes in Li-Ion Batteries”, Journal of Power Sources, 113 (1), 166-172 (2003)), while others have investigated diphenyloctyl phosphate and tris(2,2,2-trifluoroethyl)phosphate in 1.15M LiPF6 in EC+EMC (3:7 vol %) (Nam, et al., “Diphenyloctyl Phosphate and tris(2,2,2-trifluoroethyl)Phosphite as Flame-Retardant Additives for Li-ion Cell Electrolytes at Elevated Temperature”, Journal of Power Sources, 180 (1), 561-567 (2008)). Triphenylphosphite (see FIG. 5g) has been investigated in solutions consisting of 1.0M LiPF6 in EC+DEC+DMC (1:1:1 wt %) using concentrations of 10 wt % FRA (Ma, et al., “A Phosphorous Additive for Lithium-Ion Batteries”, Electrochemical and Solid State Letters, 11(8), A129-A131 (2008)).
A number of future NASA missions involving the exploration of the Moon and Mars will be “human-rated” and, thus, require high specific energy rechargeable batteries that possess enhanced safety characteristics for such “human-rated” applications. Given that Li-ion technology is the most viable rechargeable energy storage device for near term applications, effort has been devoted to improving the safety characteristics of this system. Li-ion technology has been identified as being the most promising energy storage device for near term applications, and extensive effort has been devoted to developing advanced anode and cathode materials to improve the energy density. With these new electrode chemistries, there is strong desire to develop Li-ion batteries with improved safety characteristics for both aerospace and terrestrial applications, including for automotive applications such as for hybrid electric vehicles (HEVs) and plug-in hybrid electric vehicles (PHEVs).
The current state of art lithium ion battery electrolytes consists of mixtures of organic carbonates with a lithium ion conducting electrolyte salt, such as lithium hexafluorophosphate (LiPF6). The baseline electrolyte that was employed in the work disclosed herein is a formulation that was originally developed for use in the Mars Surveyor Project 2001 Lander, and consists of 1.0M LiPF6 in EC+DEC+DMC (1:1:1 volume %) (Smart, et al., “Electrolytes for Low-Temperature Lithium Batteries Based on Ternary Mixtures of Aliphatic Carbonates”, Journal of the Electrochemical Society, 146(2), 486-492 (1999); Smart, et al., U.S. Pat. No. 6,492,064, issued Dec. 10, 2002, entitled “Organic Solvents, Electrolytes, and Lithium Ion Cells with Good Low Temperature Performance”).
This electrolyte, in conjunction with MCMB anodes and LiNiCoO2 cathodes, has been demonstrated to provide good cycle life characteristics and a wide operating temperature range (i.e., −30° to +40° C.) in large capacity cells manufactured by Yardney Technical Products, and has been used for a number of NASA past and upcoming missions, including 2003 MER Rovers, Phoenix Lander, Grail, June and the Mars Science Laboratory (MSL) Rover (Smart, et al., “Life Verification of Large Capacity Yardney Li-ion Cells and Batteries in Support of NASA Missions”, International Journal of Energy Research, 34, 116-132 (2010)). Although this electrolyte provides good performance characteristics, there is a desire to improve the safety of the system, due to the fact that abuse conditions can often lead to cell rupture and fire. It is well known that the nature of the electrolyte can greatly affect the propensity of the cell/battery to catch fire, given the flammability of the organic solvents used within. Thus, developing advanced electrolytes to reduce the flammability of cells/batteries has been studied.
To improve the safety characteristics by reducing the flammability of the electrolytes, a number of approaches have been adopted, including the use of low flammability solvents and the use of electrolyte additives. With respect to the use of electrolyte additives, the main focus has been upon the use of phosphorus containing additives, including trimethyl phosphate and triethyl phosphate, as discussed above, as well as tris(2,2,2-trifluoroethyl)phosphate (Xu, et al., “Nonflammable Electrolytes for Li-Ion Batteries Based on a Fluorinated Phosphate”, Journal of the Electrochemical Society, 149(8), A1079-A1082 (2002); Xu, et al., “Evaluation of Fluorinated Alkyl Phosphates as Flame Retardants in Electrolytes for Li-Ion Batteries, I. Physical and Electrochemical Properties”, Journal of the Electrochemical Society, 150(2), A161-A169 (2003)), bis(2,2,2-trifluoroethyl)methyl phosphonate (TFMPo) (Xu, et al., “Nonflammable Electrolytes for Li-Ion Batteries Based on a Fluorinated Phosphate”, Journal of the Electrochemical Society, 149(8), A1079-A1082 (2002); Xu, et al., “Evaluation of Fluorinated Alkyl Phosphates as Flame Retardants in Electrolytes for Li-Ion Batteries, I. Physical and Electrochemical Properties”, Journal of the Electrochemical Society, 150(2), A161-A169 (2003)), tris(2,2,2-trifluoroethyl)phosphite (Zhang, et al., “Tris(2,2,2-trifluoroethyl)Phosphite as a Co-Solvent for Nonflammable Electrolytes in Li-Ion Batteries”, Journal of Power Sources, 113, 166-172 (2003); Nam, et al., “Diphenyloctyl Phosphate and tris(2,2,2-trifluoroethyl)Phosphite as Flame-Retardant Additives for Li-ion Cell Electrolytes at Elevated Temperature”, Journal of Power Sources, 180, 561-567 (2008)), triphenylphosphite (Ma, et al., “A Phosphorous Additive for Lithium-Ion Batteries”, Electrochemical and Solid-State Letters, 11(8), A129-A131 (2008)), diethyl ethyl-phosphonate, and diethyl phenylphosphonate. Some of these electrolyte additives have been demonstrated to perform well in multi-component Li-ion battery electrolytes. For example, tris(2,2,2-trifluoroethyl)phosphate has been used in electrolyte solutions consisting of 1.0M LiPF6 in EC+EMC (1:1 wt %) in varying concentrations (Xu, et al., “Nonflammable Electrolytes for Li-Ion Batteries Based on a Fluorinated Phosphate”, Journal of the Electrochemical Society, 149(8), A1079-A1082 (2002); Xu, et al., “Evaluation of Fluorinated Alkyl Phosphates as Flame Retardants in Electrolytes for Li-Ion Batteries, I. Physical and Electrochemical Properties”, Journal of the Electrochemical Society, 150(2), A161-A169 (2003)).
Therefore, extensive effort has been devoted recently to developing non-flammable electrolytes to reduce the flammability of the cells/battery. A number of electrolytes with reduced flammability have been developed and demonstrated to be compatible with carbon based anodes and a range of cathode materials, including LiNiCoO2, LiNiCoAlO2, and LiMnNiCoO2. For example, promising electrolytes have been developed incorporating flame retardant additives and have been displayed to have good performance in a number of systems, including cells with (1) MCMB anodes and LiNiCoO2 cathodes, (2) MPG-111 graphite anodes and LiNiCoAlO2 cathode electrodes, and (3) lithium metal anodes and (Li0.17Ni0.25Mn0.58)O2 cathodes. However, these electrolyte formulations did not perform well when utilizing carbonaceous anodes with the high voltage materials, including (Li0.17Ni0.25Mn0.58)O2 and Li1.2Mn0.53Ni0.18Co0.1O2 cathodes. Thus, further development was required to improve the compatibility. In addition, although some of the electrolytes identified and previously described perform well when coupled with silicon-based anode materials, there is a desire to obtain further improved electrolytes that result in long life, while still providing enhanced safety.
In support of a NASA funded project, advanced electrolytes that have improved safety characteristics for advanced lithium-ion chemistries for “human-rated” applications were developed. In addition to resulting in improved safety, the electrolytes should provide: (i) good cycle life characteristics, (ii) good rate capability down to 0° C., and (iii) tolerance to high voltages (i.e., up to 5.0V). Although the current state of art lithium ion battery electrolytes perform well with traditional carbon anodes and low voltage cathodes, such as the baseline electrolyte discussed above in paragraph [0010] adopted for the SPS program (namely 1.0M LiPF6 in EC+DEC+DMC (1:1:1 volume %) (Smart, et al., “Electrolytes for Low-Temperature Lithium Batteries Based on Ternary Mixtures of Aliphatic Carbonates”, Journal of the Electrochemical Society, 146(2), 486-492 (1999); Smart et al., U.S. Pat. No. 6,492,064, issued Dec. 10, 2002, entitled “Organic Solvents, Electrolytes, and Lithium Ion Cells with Good Low Temperature Performance”), there is a desire to improve upon this performance with silicon anodes and high voltage cathodes. Moreover, there is a desire to reduce the flammability of this mixture, since abuse conditions may lead to cell rupture and fire. It is well accepted that the electrolyte type can greatly affect the propensity of the cell/battery to catch fire, given the flammability of the organic solvents used within.
Silicon-based alloy anode materials are especially attractive alternatives to the traditionally used carbon-based anodes, offering nearly three times more specific capacity. However, due to dramatic volume changes during cycling which results in mechanical disintegration, the materials are generally observed to have rapid capacity fading. Some groups have attempted to modify the electrolyte solution used in these systems with the intent of stabilizing the electrode-electrolyte interface, with the prospect of enhanced life. An ionic electrolyte is known that consists of 1.0M lithium bis(trifluoromethylsulfonyl)imide/1-methyl-1-propylpyrrolidinium bis(trifluoro-methylsulfonyl)imide (LiTFSI/MPP-TFSI) which was observed to provide improved cycle life of Si—Cu film electrodes compared to cells with 1.0M LiPF6 in ethylene carbonate (EC)+diethyl carbonate (DEC), being attributed to desirable solid electrolyte interphase (SEI) formation (Nguyen, et al., “Characterization of SEI Layer Formed on High Performance Si—Cu Anode in Ionic Liquid Battery Electrolyte”, Electrochemistry Communications, 12(11), 1593-1595 (November 2010)).
Researchers have reported that the use of fluoroethylene carbonate results in improved cycle life of Si electrodes, due to the formation of a stable SEI consisting of lithium fluoride and a polyene-compound (Nakai, et al., “Investigation of the Solid Electrolyte Interphase Formed by Fluoroethylene Carbonate on Si Electrodes”, Journal of the Electrochemical Society, 158(7), A798-A810 (2011)). Specifically, the researchers described an electrolyte consisting of 1.0M LiPF6 in FEC+DEC (1:1 v/v) delivered improved cycle life performance in Li/Si cells compared to 1.0M LiPF6 in EC+DEC (1:1 v/v). A known system consisting of 0.5M LiBOB+0.38M LiPF6 in EC+DMC+EMC (1:1:1 v/v %)+2% VC reported improved cycling life with Si thin-film electrodes when testing with a lithium counter electrode (Li, et al., “Electrochemical Performance of Si/Graphite/Carbon Composite Electrode in Mixed Electrolytes Containing LiBOB and LiPF6”, Journal of the Electrochemical Society, 156(4), A294-A298 (2009)). Other approaches involving the use of electrolyte additives include: (a) the use of tris(pentafluorphenyl)borane (2-5 wt %) in LiClO4 in EC+DEC (1:1 v/v) electrolytes (Han, et al., “Tris(pentafluorophenyl)borane as an Electrolyte Additive for High Performance Silicon Thin Film Electrodes in Lithium Ion Batteries”, Electrochimica Acta, 56(24), 8997-9003 (2011)), (b) the use of succinic anhydride (3 wt %) in LiPF6 in EC+DEC (1:1 v/v) electrolytes (Han, et al., “Effect of succinic anhydride as an electrolyte additive on electrochemical characteristics of silicon thin-film electrode”, Journal of Power Sources, 195(11), 3709-3714 (2010)), (c) the use of vinylene carbonate (1 wt %) in LiPF6 in EC+DMC (1:1 v/v) electrolytes (Chen, et al., “Enhancing electrochemical performance of silicon film anode by vinylene carbonate electrolyte additive”, Electrochemical and Solid State Letters, 9(11), A512-A515 (2006)). It should be noted that none of these electrolytes were designed to possess low flammability or improved safety. In addition, all of these studies were performed with lithium metal counter electrodes and the performance with high voltage cathodes was not addressed.
Accordingly, there is a need for lithium-ion electrolytes with improved safety tolerance to high voltage systems and a need for lithium-ion electrolytes with the ability to operate with high capacity silicon-based anodes and high voltage cathodes.